SiR-FP43, the NADPH- and FAD-binding domain of the Escherichia colisulphite reductase flavoprotein component (SiR-FP), has been overexpressed and characterized. It folds independently, retaining FAD as a cofactor and the catalytic properties associated with the presence of this cofactor. Iodonium diphenyl chloride (IDP) was shown to be a very efficient inhibitor of SiR-FP43 and SiR-FP60, the monomeric form of SiR-FP, containing both FMN and FAD as cofactors (Ki = 18.5±5 μM, maximal inactivation rate = 0.053±0.005 s-1). In both cases, inactivation was shown to result from covalent binding of a phenyl group to FAD exclusively, in marked contrast with previous results obtained with cytochrome P450 reductase (CPR), where FMN and a tryptophan were phenylated, but not FAD. However, our kinetic analyses are in agreement with the inhibition mechanism demonstrated with CPR [Tew (1993) Biochemistry 32, 10209-10215]. Nine different FAD phenylated adducts were isolated and, for the first time, two FAD phenylated adducts were identified directly after extraction from a protein. Taken together, our results have shown that flavoprotein inactivation by IDP is not a reliable indicator for a flavin radical intermediate in catalysis.
Overexpression of the FAD-binding domain of the sulphite reductase flavoprotein component from Escherichia coli and its inhibition by iodonium diphenyl chloride
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Jacques COVÈS, Colette LEBRUN, Gaspard GERVASI, Pascal DALBON, Marc FONTECAVE; Overexpression of the FAD-binding domain of the sulphite reductase flavoprotein component from Escherichia coli and its inhibition by iodonium diphenyl chloride. Biochem J 1 September 1999; 342 (2): 465–472. doi: https://doi.org/10.1042/bj3420465
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